One common theme is magic and its uses. Since the wizard is a central character, stories often explore different spells and how they impact the village or other characters. Another theme is friendship. The wizard usually forms friendships with the villagers, and fanfictions might show how these relationships develop.
Well, it could be about a character from Harvest Moon who is a big Chelsea fan. Maybe he or she moves to London near Chelsea's stadium. They could get involved with the local Chelsea fan community, and there could be a love story with another Chelsea fan. It might also involve how their love for the team affects their relationship.
One common theme is friendship. The relationships between the main character and the villagers or the animals are often explored in depth. For example, stories might focus on how the main character helps a sick animal and in the process, they become closer friends.
No, they're not the same. Story of Seasons is a new series that developed from the Harvest Moon franchise but has some distinct features and gameplay differences.
They aren't identical. Harvest Moon was the original, and Story of Seasons is a continuation with its own tweaks and improvements. The worlds, quests, and overall feel can vary between the two.